Method and apparatus for measuring the axial load on a valve stem

Abstract: Disclosed is a method and apparatus for measuring and monitoring the axial load or thrust on the valve stem (11) of an air operated valve assembly (1). The top end of the valve stem (11) is connected to the bottom end of the valve actuator drive shaft (4) by a marriage block connector member (14) having a plurality of strain gages (23) capable of generating an electrical signal representative of the axial load or thrust on the valve stem (11). The marriage block connector member (14) is also provided with a gage (27) capable of generating an electrical signal representative of the axial position of the valve stem (11). The apparatus and methods disclosed are used for valve diagnostic testing from a remote location or for a valve control system.

Description: This invention relates to a method and apparatus for measuring and monitoring the axial load on a valve stem used to open and close a valve. This invention is especially useful in the field of valve diagnostics. In the nuclear power industry, valves are actuated remotely by electric motors or compressed air to open, closed or intermediate positions to improve or maintain power plant output and to maintain proper cooling of the nuclear reactor. Continuous and proper operation of the valves is essential for reliable operation of the power plant and the safety of the general public. The power and process industries and the valve manufacturing industry have had a need to develop apparatus and systems to continuously measure, test and monitor various operating parameters associated with valves, especially remotely located valves, and to alert the plant operator of any significant changes in any of the operating parameters being measured, tested and monitored. Two of the important operating parameters to be measured, tested and monitored in a valve are the axial load or thrust exerted on the valve stem by the valve operating motive power, such as an electric motor or compressed air, and the axial position of the valve stem, which is also indicative of the position of the valve. The systems that have been developed in the prior art for measuring valve stem axial load or thrust, in addition to being primarily adapted for motor operated, rather than air operated valves, usually measure the valve stem axial load or thrust indirectly and therefore must be carefully calibrated for each installation and then regularly recalibrated during service to determine how the measurements relate to the actual valve stem axial load or thrust. It is therefore an object of this invention to provide a method and apparatus to permit the continuous and direct measurement and monitoring of the valve stem axial load on power operated valves, such as those powered by electric motors and solenoids, pressurized air or hydraulic actuators. This object is achieved by the invention claimed in claims 1 and 7. US Patent Specifications 4,930,228 and 4,570,903, which correspond to the pre-characterising part of claims 1 and 7, disclose techniques for measuring the axial load on a valve stem. However, US Patent Specification 4,930,228 effectively measures the change in pitch of the screw thread on the valve shaft, while US Patent Specification 4,570,903 uses a device attached at the upper free end of the valve shaft. Neither discloses the use of a marriage block capable of generating an electrical signal representing axial load, as claimed in the characterising part of claims 1 and 7. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a front elevational view, partly in section, of a typical commercially available air operated valve assembly according to the prior art. FIGURE 2 is a front elevational view, partly in section, of a typical commercially available air operated valve assembly equipped with an apparatus for measuring and monitoring the axial load or thrust on the valve stem and the axial position of the valve stem, and a valve diagnostic and control system, according to an embodiment of the invention. FIGURE 3 is an isometric "exploded" view of a typical prior art marriage block used to connect the bottom end of the valve actuator drive shaft to the top end of the valve stem that opens and closes the valve. FIGURE 4 is a sectional view of the typical prior art marriage block, illustrated in FIGURE 3. FIGURE 5 is an isometric view of the marriage block, of a preferred embodiment of this invention, used to connect the bottom end of the valve actuator drive shaft to the top end of the valve stem that opens and closes the valve. FIGURE 6 is a sectional view of the marriage block, of the preferred embodiment of this invention, illustrated in FIGURE 5. FIGURE 7 is a side elevational view of the marriage block, of the preferred embodiment of this invention illustrated in FIGURE 5. FIGURE 1 is a front elevational view, partly in section, of a typical commercially available air operated valve assembly 1. As shown in FIGURE 1, this valve assembly 1 is comprised generally of an upper pressed steel diaphragm case 2 which contains a rubber diaphragm 3 attached to a valve actuator drive shaft 4 which is moved axially downwardly by compressed air acting on the top of the rubber diaphragm 3 or axially upwardly by a spring 5 which surrounds the valve actuator drive shaft 4 within a spring housing 6 and bears against the underside the rubber diaphragm 3. A valve yoke 7 supports the spring housing 6 and the diaphragm case 2 on the valve body 8 that contains the valve. As shown in FIGURE 1, the bottom end of the valve actuator drive shaft 4 is threaded and threads into the top of a steel "marriage block" connecting member 9 and secured with a locknut 10. The top of the valve stem 11 that is connected to and operates the valve in the valve body 8, is also threaded and threads into the bottom of the marriage block 9 and is secured with a locknut 12. As shown in FIGURES 3 and 4, the valve actuator drive shaft 4 is preferably tubular with an inside diameter larger than the outside diameter of the solid valve stem 11, which allows for the top of the valve stem 11 to fit within the tubular valve actuator shaft 4 if necessary. The main purpose of the prior art marriage block connecting member 9 is to permit a simple means to adjust and match the axial stroke length of the valve actuator drive shaft 4 to the stroke length of the valve stem 11 needed to fully open and close the valve. A typical prior art marriage block 9 is made of two machined steel mating halves bolted together, as shown in FIGURES 3 and 4, which allow for easy installation and replacement of the marriage block 9 in a valve assembly 1, as required. As shown in FIGURE 1, commercial air operated valve assemblies 1, are often equipped with a vernier sight gage 13 attached to the yoke 7 and indexed to the top or bottom edge of the marriage block 9 to permit an on-site visual measurement of the axial position of the valve stem 11, but such a vernier sight gage 13 does not allow for the monitoring of the axial position of the valve stem 11 from a location remote from the valve assembly 1. FIGURE 2 is a front elevational view, partly in section, of a typical commercially available air operated valve assembly 1 equipped with the apparatus for measuring and monitoring the axial load or thrust on the valve stem 11 and axial position of the valve stem 11, according to a preferred embodiment of this invention. The apparatus of this embodiment is also illustrated in FIGURES 5-7, which illustrate a marriage block 14 to replace the conventional prior art marriage block 9, illustrated in FIGURES 1, 3 and 4, and described above. As shown in FIGURES 2 and 5-7, the marriage block connecting member 14 of this embodiment is comprised of a pair of machined steel mating halves 15 joined together with bolts 16 to resemble a cube provided with a threaded top opening and recess 17 in the top portion 21 to receive the bottom threaded end of the tubular valve actuator drive shaft 4 and a threaded bottom opening and recess 18 in the bottom portion 22 to receive the top threaded end of the valve stem 11. As best shown in FIGURE 5, two opposed sides of the cubical marriage block 14, have central openings 24 and the other two sides are each provided with a pair of slots 19 which form a pair of opposed horizontal beam members 20 connecting the top portion 21 and the bottom portion 22 of the marriage block connecting member 14. As best illustrated in FIGURE 7, the two opposed horizontal beam members 20 in the two opposed sides of the marriage block connecting member 14 of this embodiment are each provided with several electrical resistance strain gages 23 bonded to the top and bottom surfaces of the horizontal beam members 20 and electrically connected together in a wheatstone bridge configuration that will continuously and directly measure the strains in the horizontal beam members 20 connecting the top portion 21 and bottom portion 22 of the marriage block 14. This allows the marriage block 14 to act essentially as a load cell connecting the bottom of the valve actuator drive shaft 4 and the top of the valve stem 11 to permit direct and continuous measurement, monitoring, calculation and display of the axial load or thrust imposed on the valve stem 11 at all times with a great deal of accuracy. In this embodiment, electrical resistance constantan foil strain gages 23 with a gage factor of two, manufactured by MicroMeasurements Group, gave good reliable results. As illustrated in Figures 2 and 5, the marriage block 14 of this embodiment is also provided with an arm 25 preferably attached to the side of the marriage block 14, as shown in FIGURE 5, or alternatively to an exposed portion of the valve stem 11 through an opening 24 in the side of the marriage block 14. The arm 25 supports a magnetic ring or "donut" 26 that forms the movable part of a magnetostrictive position gage, of the type manufactured and sold by Rosemount Corporation. As illustrated in FIGURE 2, the magnetic ring or donut 26 surrounds a fixed elongated rod-like magnetic sensor 27 mounted to the spring housing 6 or the yoke 7 of the valve assembly 1. Any upward or downward axial movement of the valve stem 11 will cause an equivalent upward or downward movement of the arm 25 and the magnetic ring or donut 26, which movement will be instantly measured and monitored by the elongated rod-like magnetic sensor 27 as an electrical signal, thereby providing a means for the continuous and direct measurement of the axial position of the valve stem 11 at all times to a remote location. While in the preferred embodiment of this invention, a magnetostrictive gage of the type described above was used to measure the axial position of the valve stem 11, other types of position gages capable of producing an electrical signal indicating the axial position of the valve stem, such as string potentiometer, string encoders, LVDTs or glass slide encoders could be used if desired. However, the magnetostrictive type of gage, described above, is preferred due to its accuracy, permanent sensitivity and durability under most operating conditions that practically eliminate the need for any recalibration of this type of gage, while in service. The marriage block 14 of this embodiment may be installed temporarily on a valve assembly 1 for diagnostic testing of the valve assembly 1 using a conventional data collection and display means or it may be permanently installed on a valve assembly 1 to facilitate either the regular diagnostic testing of the valve assembly 1 from a remote location or used to provide valve operating parameters for a valve control system. In a valve control system for air operated valves, it may be necessary to also measure and monitor the air pressure used to operate the valve diaphragm 3 and the valve actuator drive shaft 4, by placing a pressure transducer 28 having an electrical signal output, in the compressed air line 29 leading to the valve actuator. FIGURE 2 illustrates a schematic block diagram to illustrate one such control system that utilizes a computerized data collection and controller 30, such as a Model 3000 series controller unit manufactured by ITI Movats, Inc. and electrical signals received continuously to indicate the air pressure, the axial valve stem load or thrust and the axial position of the valve stem as provided by the apparatus and method of this embodiment, as described above, and uses such signals for diagnostic testing and/or for controlling the operation of the valve assembly 1 from a remote location.

Claims: A method for measuring and monitoring the axial load on a valve stem (11) characterised by the steps of connecting the valve stem (11) to a valve actuator drive shaft (4) with a marriage block connecting member (14) capable of generating an electrical signal representative of the axial load on the valve stem (11), and using the marriage block connecting member (14) to generate electrical signals representative of the axial load on the valve stem (11). The method of Claim 1 wherein the marriage block connecting member (14) is a load cell. The method of Claim 1 or 2 which includes the additional step of generating an electrical signal representative of the axial position of the valve stem (11). The method of Claim 3 in which the electrical signals are used to diagnose the operation of a valve. The method of Claim 3 in which the electrical signals are used to control the operation of a valve. The method of any preceding Claim, in which the valve actuator drive shaft (4) is actuated by compressed air. Apparatus for measuring and monitoring the axial load on a valve stem (11) characterised by a marriage block connecting member (14) connecting the valve stem (11) with a valve actuator drive shaft (4), said marriage block connecting member (14) being capable of generating an electrical signal representative of the axial load on the valve stem (11). The apparatus of Claim 7 further including means (27) to generate an electrical signal representative of the axial position of the valve stem (11). The apparatus of Claim 8 in which the means (27) to generate the electrical signal representative of the actual axial position of the valve stem is a magnetostrictive gage. The apparatus of Claim 7, 8 or 9, wherein the valve is an air operated valve, and the apparatus further includes a means for generating a signal indicating the pressure of the air supplied to the valve.

Inventors: ANDERSON PAUL GENE, SMITH CHRISTOPHER PAUL, ANDERSON, PAUL GENE, SMITH, CHRISTOPHER PAUL
Applicants: CRANE NUCLEAR INC, CRANE NUCLEAR, INC.
Application year: 1994
Main classification: G01L5
IPC codes: G01L5